Systems used for thermal-imaging or focusing laser energy and other infrared sources, normally comprise a set of several lenses. The reason for this is that it is extremely difficult and often impossible to obtain an image or a clear focus using a single lens, due to spherical and chromatic optical distortions. In order to compensate for the distortions, entire sets of lenses, or alternatively aspherical lenses are being used. The latter have the drawback that their polishing is complicated and expensive. The increase of the number of components in the optical path of the image increases the weight and price of the system, reduces its reliability and, especially, causes loss of transmission. For example, transmission by each germanium lens is about 50%.
The use of sets of lenses for transmittance of a clear image is common practice also in the visible region. For example, any camera "lens" comprises a set of about 5 to 10 different lenses to facilitate correction of aberrations. In the IR region the traditional solution to correct optical distortions by using sets of lenses is more problematic, since the materials transparent in the IR region have high refraction indices and densities and are most expensive. The use of single optical elements instead of a set of conventional lenses increases the reliability of the system and reduces the cost thereof.
Imaging by conventional lenses is induced by a discrete refraction occurring at the boundaries of a homogeneous medium and depending on the refraction index, thicknesses and surface curvatures. The technique of graded-index optics, which is characterized by using lenses with continuously graded refraction index (GRIN) each having simpler and less costly surface shape, can do with fewer elements than an assembly of a plurality of lenses each having a uniform refraction index. Lens elements with GRIN are able to focus light without curved surfaces. This ability results from the fundamentally different way light behaves as it travels through a transparent material with GRIN as opposed to material with a uniform refraction index. Whereas in the latter light is refracted only at the surface of the lens, a lens element with GRIN bends light continuously as it travels across the lens, causing the rays to progress along a curved path.
Combining surface refraction with continuous refraction provides a number of advantages over conventional lens systems, such as correction of aberration without complex multi-element systems or aspheric elements, simplification of the geometry of the lenses and formation of real images at the lens surface.
Several methods are known for the preparation of GRIN elements for the visible region.
By one known method, glass is bombarded with neutrons to create a change in the refraction index. The major disadvantages of such technique are the large number of neutrons necessary to bring about a change of the refraction index and the possibility that the changes are reversible so that the resulting gradient is not permanent.
Another known method involves chemical vapour deposition (CVD). This technique has been widely employed in the manufacture of graded index optical fibres for use in telecommunication. Essentially, the process comprises successively depositing by CVD techniques vitreous layers of different compositions on either the inside or the outside of a silica tube. Each deposited layer has a different refraction index and in this way a radially graded refraction index is achieved. This technique has severe limitations for large-geometry lenses.
Yet another known technique is the so-called ion stuffing method which comprises heating special glass until it phase-separates. One of the phases is selectively dissolved out of the glass by means of an acid which leaves behind a sponge-like glass matrix which is immersed in a bath containing ions or molecules. The ions or molecules in the bath diffuse into the glass matrix and before saturation, the diffusion is discontinued whereby a concentration gradient is formed. In a final step, the glass is heated so as to bring about a collapse of the spongy structure which results in the formation of a lens element with a graded refraction index. This technique is cumbersome and difficult to monitor and only limited types of profiles can be produced thereby.
A still further known technique for making GRIN lens elements is based on crystal growing and this method has been shown to work with sodium chloride and silver chloride combinations. Starting with a sodium chloride seed, the crystal is pulled. With time, more sodium chloride is pulled out of the bath whereby the silver chloride concentration in the bath is increased and in this way the composition of the growing crystal and accordingly its refractive index varies gradually. It has also been suggested to grow by this technique silicon-germanium crystals with a graded refraction index for IR transmission but this proposal has not led to any commercial product.
The probably most widely used known method for the production of GRIN lens elements is the diffusion method which is the simplest in terms of instrumentation and control. By this method the glass is immersed into a bath of a molten salt, and ions such as lithium diffuse into the glass and exchange other ions therein. In the course of diffusion, a concentration gradient of the diffusing ions is formed with the concentration of the ions decreasing from the bath-glass interface into the glass and this in turn gives rise to a graded refraction index. In order to increase the refraction index, glass ions are exchanged for other ions having larger radii, i.e. ions having a greater degree of polarization for incident light. Several factors influence the rate of ion exchange, such as the electrochemical affinities between opposing ions, the bonding strength that retains the glass ions in the lattice site and relative mobilities of the various ionic species inside the glass network.
The ion mobility depends on the temperature. A high temperature can be thought of as supplying additional vibrational energy to the glass lattice and expanding the channels through which the ions are free to move. Although the temperature plays a dominant role, the glass composition is important in determining which ions can be exchanged and what physical condition the glass will be in at the end of the process.
By the above known techniques, three types of GRIN lens elements can be obtained:
(i) GRIN elements with an axial gradient - in this type of GRIN elements the index of refraction varies along the optical axis and the surfaces of constant refraction index are outer planes perpendicular to the symmetry axis. Such axial gradient GRIN lenses are particularly useful for replacing aspheric surfaces. In fact, it has been shown theoretically that there exists a 1 to 1 correlation between an axial gradient and an aspheric surface (D. T. Moore, J. Opt. Soc. Am. 68 (9) 1157-1166 (1978) and Phys. Teach. 15 (7), 409-413 (1977). Accordingly, this technique makes it possible to replace prior art aspheric surface lenses with spherical GRIN lenses having an axial gradient.
(ii) GRIN elements with a radial or cylindrical gradient - in this type of GRIN elements the refractive index varies from the symmetry axis outward and the surfaces of constant index are cylinders whose axis corresponds to the optical axis of the lens. Two notable examples of such GRIN elements are graded index optical fibres in which the length of the fibre is much greater than the diameter (J.G. Beale, Phys. Chem. Glasses 212 (1) 5-21 (1980) and the Wood lens first made in 1905 (Robert W. Wood, Physical Optics [1934, McMillan New York]).
(iii) GRIN elements with spherically graded refraction index - in this type of GRIN elements the refractive index gradient is centrosymmetrical so that the surface of constant indices are spheres. Where the gradient centre of symmetry coincides with the centre of curvature, there are obtained the so-called Maxwell Fisheye lenses (first developed in 1854) and the Luneburg lenses used in the microwave region of the spectrum and for acoustical imaging. No optical applications of this type of GRIN lenses have ever been developed.
None of the above described techniques for the manufacture of GRIN lens elements is useful for imaging in the infrared region. It is therefore the object of the present invention to provide a method for making GRIN lens elements for imaging in the infrared region of the optical spectrum and therefore to fulfill a long-felt need.